The primary objective of the study is to assess efficacy and safety of different prophylactic or therapeutic antithrombotic approaches in patients with hematologic neoplasms and platelet count <50 x109/L, including unfractionated or low molecular weight heparin, fondaparinux, anti-vitamin K agents, antiplatelet agents, novel oral anticoagulants, fibrinolytic agents, with or without a policy of platelet transfusion. Cases with arterial or venous thromboembolism managed with observation or use of vena cava filters in patients with venous thromboembolism will be included too.